The feature allows you to preview what the next call looks like. The details of the next call comes up a sidebar on the right:

How do you enable it?

What happens if an alternate number is next?
If the number to dial next is an alternate number for the current prospect, it shows as:

FAQ:

• How does this feature work?

Set the dialer to Power Dialer. You can then click the sidebar on the right and the details of the next dials will show up.  

• Can I turn it off? 

Yes. The sidebar will collapse the Preview Next section and hide it from view.

• Does this work for the parallel dialer?

This feature is for the power dialer only. With the parallel dialer, you’ll already automatically see who you’ll be calling next in the dialer screen.